Frequently, fertilizers are applied in the soil to supply plants with: primary macronutrients like nitrogen, phosphorus and potassium; secondary macronutrients like calcium, sulfur and magnesium; and micronutrients like zinc, boron, copper, manganese and molybdenum.
Nitrogen and potassium are the two nutrients for which plants have the highest demand. Potassium chloride has been the primary source of potassium for the fertilizer industry. The primary commercially available forms of potassium fertilizers are powder and granular. However, due to the high solubility of potassium chloride, a significant portion of the product is often lost by leaching, leading to a relatively low efficiency of these products.
Typically, plants require magnesium at from about 10 to 40 kg/hectare. Magnesium deficiencies occur frequently in acidic soils and are often exacerbated by high applications of potassium.
Typically, plants require 10 to 30 kg of sulfur per hectare, although some plants have a higher sulfur demand. The sulfur is often applied indirectly to the cultures, as components of some fertilizers such as superphosphate, ammonium sulfate and potassium sulfate, or plaster that is a by-product of phosphoric acid production.
Magnesium sulfate is one source of magnesium and sulfur for plants. However, magnesium sulfate is very soluble and consequently can be washed away by rainfalls. Furthermore, magnesium sulfate must be applied to the soil in multiple applications in order to provide the desired amounts of magnesium and sulfur, because a considerable portion of magnesium becomes “locked” in the soil, making it unavailable for the plants. Besides that, the application of high amounts of this product to the soil at a single time significantly elevates the salinity of the soil.
Micronutrients are agriculturally important, helping plants alleviate environmental stress, improving the nutrition quality of foods and providing higher crop production. Such micronutrients comprise boron, chlorine, copper, iron, manganese, molybdenum, nickel, and/or zinc.
Though soil conditions vary from case to case, boron-and-zinc-deprived soil conditions have frequently limited plant growth and crop production around the world. Other soil conditions where copper and other micronutrients may (also) be deprived warrant solutions as well.
How and/or how often micronutrients are applied to soil to improve the conditions are important:                Boron has a high leaching possibility, so at least the yearly application is recommended.        Copper is often complexed with an organic matter in soil.        The availability of manganese is very affected by the pH range, the microbiota and the humidity of soil.        Zinc is highly adsorbed in the clay and organic matter of tropical soil. For example, 30-60% of the adsorbed zinc may be complexed with Fe2O3 hydrate (goethite).        
Micronutrients can be conventionally supplied to plants as salts, oxides, or directly in the form of minerals, such as ulexite, colemanite, hydroboracite as an example for boron. When used in insoluble forms, the nutrients are made available through the action of organic acids produced by microorganisms present in the soil and/or by the roots of plants; these reactions are quite slow, requiring long periods of time for total use of the nutrients. The micronutrient sources are quite variable as to their physical state, chemical reactivity/bioavailability, cost and availability.
Application of micronutrients together with macronutrients or inert carriers has been done as a way to improve nutrient distribution in the soil, because the recommended doses of micronutrients per hectare are typically quite low. For example, some manufacturers developed macronutrient fertilizers with micronutrients agglutinated on the outer surface thereof which allowed for more consistent application of the nutrients to the soil. However, friction between granules during handling, transport and storage can result in the removal of the agglutinated micronutrients from the surface of these granules. On the other hand, other manufacturers combined macronutrients and soluble micronutrients by using a melting step in the manufacturing process which eliminates losses due to abrasion and segregation during application of the products. However, those products have high production cost.
Fertilizers currently used for application in the soil are typically in granular form. In comparison with powder fertilizers, granular fertilizers are easy to handle, being easily transported, stored and applied. In comparison with other products such as those in the form of pellets or granules of indefinite physical appearance and low granule size uniformity, granular fertilizers show greater fluidity and a low tendency to create dust, as a consequence of their spherical form.
One of the biggest challenges in the prior art of granulation processes is the low uniformity of the granules produced, characterized by the variability of the granules' size profiles and also by the shape of these granules, both directly impacting the moisture, the hardness and the sphericity of the finished product.
Another issue arises in the prior art with the homogenization of the mixture that is fed into the granulator. As discussed below, homogenization in the process of the present invention promotes consistency of the finished product, so that all the granules have substantially the same chemical composition and that all the ingredients (agglomerative, dispersants, rheology agent, etc.) are well dispersed and homogenized in the mixture.
The recycling of the rejected granules, that is, the granules that are outside the desired size profile, is a key part of the process of the present invention. This key part of the process is particularly essential to guarantee the uniformity of the finished fertilizer granules containing macronutrients and micronutrients. As further discussed below, this key part of the process is particularly important for granulation of potassium salts for the purpose of promoting contact between the recycled material and the material that is being granulated in the plate, leading to an increase in the hardness of the finished product.
Dust generated during the granulation process must be controlled. Depending on the materials to be granulated, the dust may cause a potentially unhealthy environment or even a potentially explosive environment.